Slow conformational dynamics of the human A_2A adenosine receptor are temporally ordered

A more complete depiction of protein energy landscapes includes the identification of different function-related conformational states and the determination of the pathways connecting them. We used total internal reflection fluorescence (TIRF) imaging to investigate the conformational dynamics of the human A_2A adenosine receptor (A_2AAR), a class A G protein-coupled receptor (GPCR), at the single-molecule level. Slow, reversible conformational exchange was observed among three different fluorescence emission states populated for agonist-bound A_2AAR. Transitions among these states predominantly occurred in a specific order, and exchange between inactive and active-like conformations proceeded through an intermediate state. Models derived from molecular dynamics simulations with available A_2AAR structures rationalized the relative fluorescence emission intensities for the highest and lowest emission states but not the transition state. This suggests that the functionally critical intermediate state required to achieve activation is not currently visualized among available A_2AAR structures.